Synergistic Autophagy and Metabolic Rate Reduction via Combined Prolonged Fasting and Nutrient-Dense Caloric Restriction

Hypothesis

In healthy adults, a regimen that alternates weekly 48‑hour water fasts with a daily 15 % caloric restriction (CR) diet rich in micronutrients and low‑glycemic carbohydrates will produce a greater increase in autophagic flux and a larger reduction in resting metabolic rate (RMR) per fat‑free mass than either intermittent time‑restricted eating (TRE) or CR alone. This synergistic effect is mediated by prolonged AMPK activation coupled with transient mTORC1 inhibition, which enhances autophagosome formation while preserving mitochondrial efficiency, ultimately lowering oxidative stress and contributing to longevity.

Mechanistic Rationale

• AMPK activation: Both fasting and CR raise the AMP/ATP ratio, activating AMPK (3). AMPK phosphorylates ULK1, initiating autophagosome nucleation, and inhibits mTORC1 via TSC2/Rheb signaling.
• Transient mTORC1 suppression: A 48‑hour fast drives a deep, though temporary, drop in intracellular amino acids, strongly inhibiting mTORC1 (1). Nutrient‑dense refeeding supplies essential amino acids to prevent maladaptive proteolysis but keeps overall ATP low enough to sustain AMPK activity.
• NAD⁺/SIRT1 axis: Fasting elevates NAD⁺, activating SIRT1, which deacetylates LC3 and ATG5, further promoting autophagosome elongation (2). SIRT1 also deacetylates PGC‑1α, enhancing mitochondrial biogenesis without increasing ROS.
• Mitochondrial efficiency: Combined AMPK/SIRT1 signaling upregulates mitochondrial genes and mtDNA copy number (3), improving oxidative phosphorylation efficiency and lowering proton leak, which reduces basal energy expenditure.
• Outcome: The integrated effect yields enhanced autophagic clearance of damaged proteins and organelles while lowering RMR per kg fat‑free mass beyond the modest ~12 % CR effect seen in CALERIE (4).

Testable Predictions

1. Autophagy flux (measured by LC3‑II/I ratio and p62 degradation in peripheral blood mononuclear cells after an overnight fast) will be significantly higher in the combined fasting+CR group than in TRE‑only or CR‑only groups after 12 weeks.
2. RMR per fat‑free mass, assessed via indirect calorimetry, will decrease by ≥8 % in the combined group, whereas TRE and CR alone will show ≤4 % reductions.
3. Core body temperature and 24‑hour energy expenditure will show parallel declines, reflecting a coordinated metabolic downshift.
4. Mitochondrial DNA copy number and SIRT1 activity will be elevated, indicating enhanced mitochondrial quality control.
5. Inflammatory markers (CRP, IL‑6) will improve to a greater extent than with either intervention alone, linking autophagy to reduced inflammaging.

Experimental Design (for falsification)

• Population: 120 healthy adults aged 30‑50, BMI 22‑27, stratified by sex.
• Arms (n=40 each):
  1. Control: ad libitum Western diet.
  2. TRE: 16:8 daily fasting, isocaloric nutrient‑dense diet.
  3. CR: 15 % daily caloric restriction, same nutrient‑dense composition.
  4. Combined: weekly 48‑hour water fast + 15 % CR on non‑fast days, nutrient‑dense diet.
• Duration: 16 weeks, with measurements at baseline, 8 weeks, and 16 weeks.
• Primary outcomes: autophagic flux (LC3‑II/I, p62), RMR/FFM, core temperature.
• Secondary outcomes: mtDNA copy number, NAD⁺/SIRT1 activity, inflammatory cytokines, lipid profile.

If the combined arm fails to show a statistically significant superiority in both autophagy flux and RMR reduction compared to the single‑intervention arms, the hypothesis is falsified. Conversely, observing the predicted synergistic improvements would support the notion that alternating prolonged fasting with moderate, nutrient‑dense CR uniquely amplifies longevity‑related metabolic and cellular maintenance pathways.